Guidelines for the Use of Proton Donors in SmI2 Reactions: Reduction of α-Cyanostilbene
Guidelines for the Use of Proton Donors in SmI2 Reactions: Reduction of α-Cyanostilbene
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DOI：
10.1021/ja9013997
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发表时间：
2009-06-17
影响因子：
15
通讯作者：
Hoz, Shmaryahu
中科院分区：
文献类型：
--
作者：
Amiel-Levy, Mazal;Hoz, Shmaryahu
在四氢呋喃中，研究了在不同质子供体存在下，SmI2还原一系列α-氰基二苯乙烯的反应。在醇TFe、i-Proh和t-BuOH存在下不发生反应。在甲醇、水和乙二醇存在下，反应发生；但在水和乙二醇存在下，反应速度太快，无法进行动力学测定(tau(1/2)和lt；1ms)。与甲醇的反应在SmI2中为一级反应，在底物中为一级反应。甲醇的顺序随其浓度的变化而变化，并且logk与log[MeOH]的关系曲线呈S型。动力学同位素效应和掺入同位素效应的比较表明，与直觉相反的是，自由基阴离子的质子化发生在碳-β到氰基上。结论是，与SmI2形成络合物的质子给体扩大了SmI2可以还原的底物范围。这是因为它们在形成时与自由基阴离子很接近。这种短命的自由基阴离子不能被主体介质中的质子供体有效地捕获。本文提出了一个关于何时应该使用与SMI_2络合的质子给体，例如MeOH、水和乙二醇，以及何时建议使用非络合质子给体，例如TFE、ProH和t-BuOH来引发反应的方案。
The reduction of a series of alpha-cyanostilbenes with SmI2 was studied in THF in the presence of various proton donors. No reaction occurred in the presence of the alcohols TFE, i-PrOH and t-BuOH. In the presence of MeOH, water and ethylene glycol the reactions occurred; however in the presence of water and ethylene glycol they were too fast for kinetic determinations (tau(1/2) < 1 ms). Reactions with MeOH were first order in SmI2 and first order in the substrate. The order in MeOH varies as a function of its concentration and the plot of log k vs log [MeOH] is sigmoidal. Comparison of the kinetic isotope effect and the incorporation isotope effect suggests that, counterintuitively, protonation of the radical anion takes place on the carbon beta to the cyano group. It is concluded that proton donors that form complexes with SmI2 expand the range of substrates that can be reduced by SmI2. This is due to their proximity to the radical anion as it is formed. This short-lived radical anion cannot be efficiently trapped by a proton donor from the bulk medium. A protocol is herein suggested as to when proton donors which Complex to SmI2, e.g. MeOH, water and ethyleneglycol should be used, and when it is recommended to use noncomplexing proton donors, e.g. TFE, PrOH and t-BuOH, to induce reaction.